SkPicture should have an offset/origin

Issue description

There are use cases (such as SkPictureImageFilter and Blink's
DisplayList) where the picture contains primitives which have a non-0,0
origin. They apply an offset transform at record time, and the inverse
transform at playback time.
This seems common enough that it might be useful to be able to specify
an origin for SkPicture, so that any tiling performed on the contained
primitives starts at that origin rather than 0,0.
It could be a constructor which takes an SkIRect, or a setOrigin/setOffset
on SkPicture.
Failing that, we should add an offset to SkPictureImageFilter.